The present invention involves microwave cooking. More particularly, the present invention is a susceptor structure for use in a microwave oven.
Heating of foods in a microwave oven differs significantly from heating of foods in a conventional oven. In a conventional oven, heat energy is applied to the exterior surface of the food and moves inward until the food is cooked. Thus, food cooked conventionally is typically hot on the outer surfaces and warm in the center.
Microwave cooking, on the other hand, involves absorption of microwaves which characteristically penetrate far deeper into the food than does infra red radiation (heat). Also, in microwave cooking, the air temperature in a microwave oven may be relatively low. Therefore, it is not uncommon for food cooked in a microwave oven to be cool on the surfaces and much hotter in the center.
However, in order to make the exterior surfaces of food brown and crisp, the exterior surfaces of the food must be heated to a sufficient degree such that moisture on the exterior surfaces of the food is driven away. Since the exterior surfaces of the food cooked in a microwave oven are typically cooler than the interior of the food, it is difficult to brown food and make it crisp in a microwave oven.
In order to facilitate browning and crisping of food in a microwave oven, devices known as susceptors have been developed. Susceptors are devices which, when exposed to microwave energy, become very hot. By placing a susceptor next to a food product in a microwave oven, the surface of the food product exposed to the susceptor is surface-heated by the susceptor. Thus, moisture on the surface of the food is driven away from the surface of the food and the food becomes crisp and brown.
Many conventional susceptor structures have included a thin metal film, typically aluminum, deposited on a substrate such as polyester. The metalized layer of polyester is typically bonded, for support, to a support member such as a sheet of paperboard or corrugated paper.
Conventional susceptors, however, have certain drawbacks. They undergo a process referred to herein as breakup in which the electrical continuity of the thin metal film is lost during cooking. The result of the loss of electrical continuity is an irreversible loss in the susceptor's microwave responsiveness and a lower level of percent power absorption by the susceptor during cooking. Lower power absorption leads to lower susceptor cooking temperatures and a corresponding decrease in the susceptor's ability to crisp food.
The susceptor's ability to crisp food is particularly hampered when the susceptor undergoes breakup prior to reaching a temperature which is sufficient to drive moisture from the surface of the food. The substrates of typical prior art susceptor structures were formed of Polyethylene Terephthalate (PET). The metalized layer was typically aluminum deposited on the PET layer. These susceptors typically underwent breakup at approximately 200.degree. C. In many cases, this is inadequate to properly surface heat food to achieve desired crisping and browning.
Thus, other materials have been tried as the substrate in susceptor structures. For example, Polyetherimide (PEI) has been metalized and used as a susceptor. When these susceptors are coupled to a support member such as cardboard, the paperboard scorches and chars because the susceptor undergoes breakup at an elevated temperature.
The foregoing discussion shows that susceptors are functional because of two seemingly similar but different principles. Susceptors heat because they absorb microwave energy which is converted to heat energy. The amount of microwave energy absorbed by susceptors depends on the surface impedance of the susceptor.
In addition to heating through absorption of microwave energy, susceptors must possess a temperature limiting feature to prevent the susceptor from over heating and scorching paper, food or other things in contact with the susceptor.
For these reasons, there is a continuing need for the development of a susceptor structure which is capable of reaching and maintaining cooking temperatures suitable for crisping and browning food products, but which also has a temperature control mechanism to avoid runaway heating conditions.